This invention relates to a contactless ignition system provided with a dwell angle control device for igniting an internal combustion engine, especially, that used for driving an automotive vehicle.
A contactless ignition system of this kind is disclosed in, for example, U.S. Pat. No. 3,605,713. FIG. 1 shows the waveform of primary current i.sub.c supplied to the ignition coil in such a disclosed ignition system. The disclosed system includes closed-loop control means so that the primary current i.sub.c supplied to the ignition coil until immediately before the generating timing of spark ignition voltage across the ignition coil can be maintained at a predetermined current level i.sub.co for a controlled period of time T.sub.i of a predetermined value as shown in FIG. 1. When, for example, the period of time T.sub.i exceeds the predetermined value, the starting timing of primary current supply to the ignition coil is delayed so as to shorten the duration T.sub.ON of current supply to the ignition coil thereby maintaining the period of time T.sub.i at the predetermined value. Such a manner of feedback control is continuously carried out to control the duration T.sub.ON of primary current supply to the ignition coil, hence, to control the dwell time so that the period of time T.sub.i can always be stably maintained at the predetermined value.
In FIG. 1, T.sub.c =(T.sub.ON -T.sub.i) represents the period of time or rising time required for the primary current i.sub.c supplied to the ignition coil until it rises to its predetermined level i.sub.co from its zero level. It is known that the non-uniformity of inductance components of ignition coils within manufacturing tolerances, during manufacture of a lot of such coils, appears directly as a corresponding non-uniformity of the length of the rising time T.sub.c. In the aforementioned prior art ignition system, a T.sub.i feedback function is provided for comparing the detected actual value of T.sub.i with its reference value T.sub.io so that the duration T.sub.ON of primary current supply to the ignition coil can be controlled depending on the error .DELTA.T.sub.i =T.sub.i -T.sub.io. Thus, when, for example, the inductance of the ignition coil employed in the ignition system is lower by 10% than the designed setting, and consequently, the rising time T.sub.c is shorter by 10% than the designed setting, the duration T.sub.ON of primary current supply to the ignition coil must also be selected to be shorter by about 10% than the designed setting.
In the prior art control system, the duration T.sub.ON of primary current supply to the ignition coil is determined depending on the error .DELTA.T.sub.i =T.sub.i -T.sub.io regardless of the difference in the inductance of each ignition coil. Therefore, when the inductance of the ignition coil is lower than the designed setting due to the manufacturing tolerances, T.sub.c will become shorter and T.sub.i will become substantially longer within the determined duration T.sub.ON of primary current supply to the ignition coil. This means that excessive heat is generated in the output stage transistor and also in the ignition coil resulting in a great temperature rise of these elements. Because of such non-uniformity of the inductance, the heat radiating fins of the output stage transistor had to be sized to be considerably larger than the size calculated according to the indexes of standard heat generation in the contactless ignition system, resulting in a difficulty of attaining the desired miniaturization of the output stage transistor. When, on the other hand, the inductance of the ignition coil is higher than the designed setting, T.sub.c will become longer and T.sub.i will become substantially shorter within the determined duration T.sub.ON of primary current supply to the ignition coil. The ignition system including such an ignition coil has been defective in that the primary current i.sub.c supplied to the ignition coil will not attain the predetermined level i.sub.co in a worst case resulting in impossibility of exhibition of the desired spark ignition performance.